Step-by-step:
The Percentage Impervious value defines the proportion of the catchment area that has been developed as roads, roofs, and other hard-standing areas. It is assumed that only rainfall landing on these areas will run off overland and end up in a manhole.
Typical housing or urban development catchment areas consist of:
- roads ≈ 30%
- roofs ≈ 30%
- permeable land ≈ 40% (from which there is assumed to be no contribution to the runoff)
Roads + Roofs = Percentage Impervious (PIMP):
- may be an assumed value
- may use digitized data for a more accurate analysis
- may employ GIS systems
